Project Summary: Cortical circuits are composed of a complex network of many neuron types. Their function is dependent on how neurons are interconnected, how the connections function, and how the information delivered to individual neurons is integrated within the postsynaptic dendritic arbor. Despite extensive knowledge of the basic blueprint of cortical circuits, detailed knowledge about precisely which cell types are connected and how the multitude of connections onto a single neuron interacts is limited. The studies proposed here will reveal the laminar and fine-scale specificities of functional excitatory and inhibitory inputs to specific types of inhibitory neurons in the cerebral cortex. A novel laser scanning photostimulation method will be used to stimulate neurons that might make connections to neurons of interest, while recording electrical responses in those neurons to determine whether connections are present. With photostimulation it is possible to stimulate hundreds of sites to "map" the sources of functional input to a single neuron. Combining this method with paired intracellular recordings and cross-correlation analyses of postsynaptic currents allows identification of shared inputs on a fine scale. Results from these studies will reveal similarities and differences in the input to different types of inhibitory neurons. Interactions between excitatory and inhibitory inputs will be tested by combining photostimulation based mapping of input patterns with stimulation of a single inhibitory neuron that is connected to an excitatory neuron of interest. This will directly test the ability of inhibition to shape the sources of excitation to a single neuron. Relevance: Understanding the detailed organization of cortical circuits involving specific inhibitory neuron types is necessary to obtain a mechanistic understanding of the function of the cerebral cortex. Understanding the specific roles of inhibitory neurons in cortical function has important implications for human health, as these cell types and their activities are implicated in the cortical mechanisms that regulate attention and their disruption is implicated in schizophrenia.